The determination of the mechanical properties of the respiratory system of a patient whose respiration is assisted by means of a respirator, which properties are characterized by the values of the compliance C and the resistance R as well as by an indicator of the activity of the respiratory muscles, remains an unsolved problem if the patient's spontaneous breathing is not negligible.
The so-called Super-Syringe process was described in the scientific literature (D. Matamis et al., Chest, 86, 1: 58 (1984)) for relaxed patients. The patient is separated from the respirator and the patient's lungs are slowly expanded by pressing in gas by means of a syringe. The airway pressure and the tidal volume pressed into the lungs are thus measured simultaneously. Thus, this process furnishes the pressure-volume relationship of the respiratory system under quasi-static conditions.
The drawback of this process is the long time needed for the measurement, during which the measured values may be distorted by the gas exchange occurring simultaneously. Furthermore, the patient must be separated from the respirator, which leads to a disturbance in respiration. The process is complicated and requires separate sensors for pressure and volume, and it is therefore used for scientific purposes only. This process cannot be used, in principle, in spontaneously breathing patients.
Sydow et al. (Intensive Care Med. (1991), 17: 108) describe a process in which the respiration is interrupted at a predetermined point by means of rapidly switching valves (this procedure is called occlusion). After pressure equalization, the pressure present in the lungs in measured. The interruption is performed one after another during a plurality of breaths, and a waiting time, during which the lungs can again assume their initial state, is inserted between the individual interruptions.
This process fails when the patient is not relaxed. A determination of the mechanical properties of the respiratory system is thus impossible in the presence of spontaneous breathing.
Furthermore, a process has been known for the determination of the respiration drive, in which the inspiration valve is closed before the beginning of a spontaneous inspiration (EVITA User's Manual, Dragerwerk, Lubeck, e.g., GA EVITA 4, July 1994 edition, p. 127). The pressure drop during the first 100 msec of the inspiration, the so-called PO.1, is evaluated as an indicator of the respiration drive. However, this process does not make it possible to determine the spontaneous breathing during the entire cycle. Furthermore, it does not make it possible to determine the mechanical properties of the respiratory system.
Kreit et al. (Am. J. Respir. Care Med.(1994), 149: 1085) described a mathematical method for the analysis of spontaneous breathing during assisted respiration, wherein the force of the respiratory muscles is represented by the so-called muscle pressure P.sub.mus. If the compliance C and the resistance R are known, the muscle pressure P.sub.mus can be calculated by means of the known mathematical relationship between the respiration pressure P.sub.aw, the muscle pressure P.sub.mus, the tidal volume V and the respiratory flow dV/dt: EQU P.sub.mus +P.sub.aw =R* dV/dt+V/C (1) EQU P.sub.mus =R* dV/dt+V/C-P.sub.aw (1a)
Three of the six factors of this equation can be measured (P.sub.aw, dV/dt and V). The authors assume the mechanical properties (compliance and resistance) of the respiratory system to be known by using for them values which had been measured before the beginning of the spontaneous breathing. This makes it possible to calculate P.sub.mus.
The drawback of this process is that its use requires the knowledge of R and C, but these variables can be determined only in the absence of spontaneous breathing. Changes in the mechanical properties of the respiratory system, which usually occur during respiration, thus inherently lead to errors in the determination of P.sub.mus. Furthermore, as is apparent from Equation (1a), P.sub.mus is determined by subtracting relatively high values. Thus, small errors in measurement thus lead to a relatively great error in the determination of P.sub.mus.
A device for monitoring the activity of the respiratory muscles of a respirated patient is described in EP 521 515. The muscle pressure P.sub.mus is calculated from the measured values of the respiration pressure .sub.Paw and the respiratory flow dV/dt according to the following formula: EQU P.sub.mus =-P.sub.aw +R* dV/dt+E* ##EQU1## with the elastance E=1/C.
This device also has the drawback that the values of resistance and compliance (or elastance) must be known. To measure these values, the patient's spontaneous breathing must be suppressed by temporarily paralyzing the respiratory muscles.
These prior-art processes and devices have the drawback that the patient's spontaneous breathing must be massively disturbed to determine the mechanical properties of the respiratory system of a patient.